The present invention relates to chromophores with large, effective two-photon absorption cross-sections.
Two-photon or multiphoton absorption occurs through the simultaneous absorption of two or more photons via virtual states in an absorbing medium, with the former being more common. For a given chromophore, these absorption processes take place at wavelengths much longer than the cut-off wavelength of its linear (single-photon) absorption. In the case of two-photon absorption (2PA), two quanta of photons may be absorbed from a single light source (degenerate 2PA) or two sources of different wavelengths (non-degenerate 2PA). Although multiphoton absorption processes have been theoretically described in 1931 and experimentally confirmed about 30 years later, this field remained dormant largely due to the lack of materials with sufficiently large two-photon sensitivity, quantified as two-photon cross-section (σ2′), which is usually expressed in the units of Göppert-Mayer (1 GM=10−50 cm4·s·photon−1·molecule−1). Then, in the mid-1990s, several new classes of chromophores exhibiting very large effective σ2′ values were reported. In conjunction with the increased availability of ultrafast high-intensity lasers, the renewed interest has not only sparked a flurry of activities in the preparation of novel dye molecules with enhanced σ2′ values, but also in advancing many previously conceived applications based on 2PA process in photonics and biophotonics, which are now enabled by these new chromophores. It is important to recognize the following useful features of the 2PA phenomenon based on the fact that 2PA scales nonlinearly with the squared intensity of the incident laser beam: (a) upconverted emission, whereby an incident light at lower frequency (energy) can be converted to an output light at higher frequency, for instance, near infrared (NIR) to ultraviolet (UV) upconversion; (b) deeper penetration of incident NIR light (into tissue samples, for example) than UV light that also may be hazardous with prolonged exposure; (c) highly localized excitation as compared with one-photon processes allowing for precise spatial control of in situ photochemical or photophysical events in the absorbing medium, thereby minimizing undesirable activities such as photodegradation or photobleaching; and (d) fluorescence, when properly manipulated, that would allow for information/signal feedback or amplification in conjunction with other possible, built-in effects such as surface plasmonic enhancement. It is anticipated that further ingenious utilization of these basic characteristics will lead to practical applications other than the ones that have already emerged in such diverse areas as bio-medical fluorescence imaging, data storage, protection against accidental laser damage, microfabrication of microelectromechanical systems (MEMS), photodynamic therapy, etc. In the past decade or so, significant advances have been made in the fundamental understanding of general structure-property relationship that has led to the design and synthesis of two-photon absorbers with very large cross-section values. Although further enhancement of 2PA cross-section is still possible as suggested by a number of theoretical studies, for certain applications, the two-photon-property requirement has essentially been met by the state-of-art chromophores. Because of the possible property-processing/fabrication trade-off, the secondary properties, e.g. thermal and mechanical properties, which are important to material processing into various useful forms (films, coatings, fibers, windows etc.) and configurations, should be addressed. For the aforementioned solid forms, polymers can offer many advantages such as the flexibility in fine-tuning the material properties and the availability of many processing options.
Accordingly, it is an object of the present invention to provide new two-photon absorbing aromatic diamines, which contain electron-donating triarylalkylamine and electron-accepting benzothiazole, and are useful monomers in the preparation of high performance polymers such as polyimides, polyamides and poly(amide-imides) for nonlinear optical applications.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The general synthetic scheme of the dinitro precursors and the corresponding diamino monomers is depicted in Scheme 1. It is known that aromatic diamines can serve as the common co-monomers for polyimide, polyamides, and poly(amide-imides). In addition, the parent compound (2a, R═H, see Scheme 1) designated as AF-240 has a relatively high effective two-photon cross-section (effective (nanosecond) σ2′ value of 9,800 GM at 800 nm) and a number of structurally related, monofunctionized (see U.S. Pat. No. 7,067,674) and difunctionalized (see U.S. Pat. No. 7,319,151) derivatives are also highly two-photon active. Based on this structural motif and together with the molecular symmetry consideration to avoid unequal reactivity, the required diamines in the present invention were designed with functionalization taking place at the 3,3′- and 4,4′-positions of the diphenylamino (donor) segment of AF240. The important intermediates to the targeted momoners, namely the 3,3′-bis(phenol) and 4,4′-bis(phenol), compounds 4c and 4 d respectively in Scheme 1, were synthesized via (i) a Pd-catalyzed amination from the fluorenyl bromide 1 and 3,3′-dimethoxydiphenylamine or 4,4′-dimethoxydiphenylamine, followed by (ii) demethylation with liquid pyridinium chloride. The diamino monomer was synthesized from the (iii) double aromatic substitution reaction of 3a or 3b and 4-fluoro-1-nitrobenzene in the presence of potassium carbonate, followed by (iv) catalytic reduction with dihydrogen or hydrazine hydrate.
